The significant role that anaerobic bacteria play in suppurative infection in man has beeen firmly established. Among all the anaerobic species, Bacteroides fragilis stands out as the most importnat pathogen because of its frequency of isolation and resistance to many antimicrobial agents. Despite the voluminous literature on B. fragilis, little is known about its pathogenic mechanisms. Current research on virulence factors have been hampered by an inability to distinguish strain differences within B. fragilis and by the lack of a genetic system. Recent studies have provided the genetic tools to facilitate a systematic invesigation of potential virulence factors in B. fragilis. The objective of this proposal is to apply the techniques of molecular biology employed for the study of aerobic organisms to define the determinants of virulence in B. fragilis. The isolates selected for study will fit the phenotype description of the type strain of B. fragilis. Transferable drug resistance will be further characterized by studying the 2 megadalton clindamycin resistance plasmid and the larger transfer factor. A transformation system will be developed utilizing the clindamycin plasmid. Characteristics which have been suggested as virulence factors will be studied by obtaining derivatives of pathogenic strains lacking the attribute and testing these clones in a rat model of intraabdominal sepsis. Initial phenotypic characteristics to be studied are the polysaccharide capsule, superoxide dismutase, catalase, extracellular nucleases and lipases. Finally, the epidemiology of B. fragilis infections will be defined using the technique of phage typing. Virulent bacteriophage of B. fragilis will be utilized to develop the typing system to determine if there are specific species associated with infections in different parts of the country and with specific sites in the body. Phage types in the normal flora will be examined. The effect that hospitalization and preoperative antibiotics have on phage types will be studied. These studies will give some insight on whether B. fragilis infections are endogenous or nosocomial.